Question

On the basis of a cold air-standard analysis, show that the back work ratio of an ideal air-standard Brayton cycle equals the ratio of absolute temperatures at the compressor inlet and the turbine outlet.

Short answer

BWR = \frac{T_1}{T_4}

Step-by-step solution

Understand the Brayton Cycle

The Brayton cycle consists of four processes: two isentropic (adiabatic and reversible) and two isobaric (constant pressure). The processes are: 1-2 isentropic compression, 2-3 isobaric heat addition, 3-4 isentropic expansion, 4-1 isobaric heat rejection.

Calculate Compressor Work

The work done by the compressor (W_c) in an isentropic process can be represented as: \[ W_c = \frac{k}{k-1} \times P_1V_1 \times \bigg[ \bigg( \frac{P_2}{P_1} \bigg)^{\frac{k-1}{k}} - 1 \bigg] = C_p (T_2 - T_1) \]

Calculate Turbine Work

Similarly, the work done by the turbine (W_t) in an isentropic process is: \[ W_t = C_p (T_3 - T_4) \]

Relate Temperatures and Pressures

Using the isentropic relations, we have: \[ \frac{T_2}{T_1} = \bigg( \frac{P_2}{P_1} \bigg)^{\frac{k-1}{k}} \] and \[ \frac{T_3}{T_4} = \bigg( \frac{P_3}{P_2} \bigg)^{\frac{k-1}{k}} \]

Express Back Work Ratio

The back work ratio (BWR) is the ratio of the work required by the compressor to the work produced by the turbine: \[ BWR = \frac{W_c}{W_t} = \frac{T_2 - T_1}{T_3 - T_4} \]

Simplify Using Temperature Relationships

Using the temperature relations from isentropic processes, we deduce: \[ \frac{T_2 - T_1}{T_3 - T_4} = \frac{T_1}{T_4} \] Therefore, \[ BWR = \frac{T_1}{T_4} \]

Key concepts

Isentropic Processes

The Brayton cycle involves two isentropic processes: one during compression and one during expansion. Isentropic means no heat transfer occurs, and the process is reversible. This aids efficiency in systems like jet engines.
In isentropic compression (process 1-2), the gas's pressure increases while volume decreases, causing an increase in temperature. Mathematically, this can be described as \(\frac{T_2}{T_1} = \bigg( \frac{P_2}{P_1} \bigg)^{\frac{k-1}{k}}\).
In isentropic expansion (process 3-4), the opposite happens: the gas does work on the surroundings, decreasing its pressure and temperature. It's expressed similarly: \(\frac{T_3}{T_4} = \bigg( \frac{P_3}{P_2} \bigg)^{\frac{k-1}{k}}\).

Compressor Work

The compressor's job is to increase the pressure and temperature of the air entering the system. In isentropic compression, the compressor work (\tW_c) is essential to calculate. The formula for compressor work is:
\[ W_c = C_p (T_2 - T_1) \]
This equation means the compressor work depends directly on the heat capacity at constant pressure (\tC_p). Understanding this helps in determining energy requirements for compressors in practical applications such as power plants and aircraft engines.

Turbine Work

In the Brayton cycle, the turbine's task is to expand the high-pressure gas, converting thermal energy into mechanical work. The work done by the turbine (\tW_t) during the isentropic expansion is given by:
\[ W_t = C_p (T_3 - T_4) \]
This tells us that the turbine work output is related to the heat capacity and the temperature difference between states 3 and 4. Turbines extract energy from the gas, which is critically important in generating power for electricity or propulsion systems.

Back Work Ratio

The back work ratio (BWR) is a critical measure of efficiency in the Brayton cycle. It is defined as the ratio of the compressor work to the turbine work:
\[ BWR = \frac{W_c}{W_t} \]
Using the temperature relationships from isentropic processes, this becomes:
\[ BWR = \frac{T_2 - T_1}{T_3 - T_4} = \frac{T_1}{T_4} \]
This indicates that higher efficiency is achieved when the temperature at the turbine outlet (T4) is much lower relative to the compressor inlet (T1), implying lower compressor requirements.

Temperature-Pressure Relationships

In the Brayton cycle, understanding the relationship between temperature and pressure is key. These relations, specifically under isentropic conditions, allow us to link different states of the cycle:
\[ \frac{T_2}{T_1} = \bigg( \frac{P_2}{P_1} \bigg)^{\frac{k-1}{k}} \]
and
\[ \frac{T_3}{T_4} = \bigg( \frac{P_3}{P_2} \bigg)^{\frac{k-1}{k}} \]
These equations show that temperature ratios are directly tied to pressure ratios across the compression and expansion processes, illuminating how pressure differences drive changes in temperature crucial to the cycle's efficiency.